The present invention relates generally to a system for controlling the operation of a gas turbine engine. More specifically, the present invention relates to the closed loop control of the working line (W/L) level of a compressor running at a constant speed through modulation of the variable inlet guide vanes (VIGV).
Designers of axial flow compressors for gas turbine engines must consider many issues associated with fluid flow including stall and/or surge. A stall generally refers to a breakdown in fluid flow in only some of the stages in a multi-stage compressor and a surge generally refers to a complete breakdown of smooth fluid flow through the compressor and generally includes the reversal of flow.
The air flow and pressure ratio of the compressor at which a surge occurs is labeled a surge point. A surge point is a characteristic of each compressor speed, and a line which joins a group of surge points drawn on a graph of pressure ratio vs. mass flow, is called the surge line. The surge line represents the minimum stable air flow which can be obtained at any rotational speed. Compressors are generally designed to have a surge safety margin between the air flow and pressure ratios at which they will normally be operated and the air flow and pressure ratios at which a surge will occur.
Many prior gas turbine engines have utilized control systems in attempts to maintain a desired surge safety margin. With reference to FIG. 1, there is illustrated a generic characteristic of a prior system including a low-pressure compressor running at a constant speed. The graph set forth in FIG. 1, includes a plurality of constant speed lines 10. More specifically, the constant speed lines 10 have substantially the same values but each is represented at a different VIGV angle. The working line 13 runs along speed line 10 between the idle point 14 and the bleed off valves (BOV) closing point 15. The position of the working line 13 between idle point 14 and the BOV closing point 15 is controlled in open loop by the modulation of the BOVs. The state of the BOVs is scheduled against corrected output power and the corrected speed of the engine.
From the BOV closing point 15 to the base load power point 16, the working line 13 position continues to be controlled in open loop through the low pressure VIGVs against corrected output power and the corrected speed of the engine. In order to avoid entering the stall/surge domain, the compressor is provided with a pressure ratio limiter 11, which monitors the working line 13 position in closed loop. Upon the working line 13 position exceeding the pressure ratio limiter 11, the control system modulates the BOVs in order to maintain the working line 13 level at the pressure ratio limiter level 11.
The surge margin is depicted in FIG. 1 between the pressure ratio limiter 11 and the surge line 12. The available surge margin is comprised of various threats, including: production scatter associated with engine to engine variation; engine deterioration and fouling; variables mal-scheduling; transient excursions above the steady state working line; and instrument accuracy. These threats erode the surge margin progressively from both sides; up by increasing the working line position level and down by lowering the surge line level. As the engine deteriorates the compressor working line position moves up progressively.
In this prior system as the working line approaches the pressure ratio limiter the BOVs are opened which results in the engine losing power and efficiency. Further, in this prior system the position of the VIGVs is scheduled against power and as the engine power drops the VIGVs angle closes progressively, further effecting engine performance and leading to further engine deterioration.
Many prior systems include techniques for controlling the operating parameters of compressors. However, there remains a substantial need for additional development in controlling the operation of a gas turbine engine compressor. The present invention satisfies this and other needs in a novel and unobvious way.